The mirror definitely is one as it really doesn't give any clues that it is in fact selectable. Another is the slot for the terminal and the pipes. With the pipes the puzzle is given to you, but it's quite a specific location. All three fall prey to the point and click troupe of trying to not make the puzzle obvious, but at the same time not giving any clues. For example, with the mirror, perhaps it could have had a crack in it, or been at a slight angle, just to signify it's 'different'. By putting it where it is, on a selectable item, it makes it seem like there's nothing else there to select, if you understand my view.